The objective is to describe the development of Babesia microti in its tick vector (Ixodes dammini) and to identify factors that affect maturation of this piroplasm. We shall determine 1. whether particular vertebrate erythrocytes may contain organisms uniquely infectious for the vector; 2. whether the piroplasm multiplies in the gut of the vector; 3. when salivary glands initially are invaded; 4. whether hormones may regulate maturation in the vector; 5. when the vertebrate host becomes infected; 6. what tissue of the vertebrate host is initially invaded; 7. whether ticks may inherit infection. This work will depend largely on electron microscopy and techniques of tick physiology. These basic studies will contribute to an understanding of the transmission of human babesiosis in New England and of piroplasmosis, world-wide.